Chemistry of nitrenium ions. IX. Discrete existence of singlet and triplet nitrenium ions

Abstract

Sir :Although the principles of nitrenium ion (divalent electron-deficient nitrogen) theory have proven invaluable in predicting the reaction products which result from the heterolytic cleavage of the N-X bond,2 no evidence existed to indicate whether a nitrenium ion could have a discrete existence. We wish to present preliminary findings which not only show that divalent electron-deficient nitrogen species are true intermediates, but also indicate that nitrenium ions exist in both singlet and triplet spin states.When N-chloramines (1) are solvolytically ionized, small amounts of secondary amine salts (2) are often observed among the reaction products.Ib For instance, 1 2 in the methanolysis of 4,7,7-trimethyl-2-chloro-2-azabicyclo[2.2. Ilheptane (3) we found 59 % 4 , 2 0 z 5 , and 7 6. Although the formation of 4 and 5 was to be expected on the basis of nitrenium ion theory, the presence of 6 appeared to be anomalous because 6 is not the type of product expected from a positively charged species. Ib CH,However, an important factor which requires consideration is that the nitrenium ion differs from most positive ions of interest to the organic chemist in that it possesses a nonbonding pair of electrons. Thus an attractive (1) Paper IX in a series on the chemistry of nitrenium ions. For the preceding papers see (a) P. G. Gassman and A. Carrasquillo, Chem. Commun., 495 (1969); (b) P. G. Gassman and R. L. Cryberg, J. Am. Chem. Soc., 91, 2047(1969.(2) P. G. Gassman and D. I<. Dygos, ibid., 91, 1543Dygos, ibid., 91, (1969; P. G.Gassman, G. Campbell, and R. Frederick, ibid., 90, 7377 (1968), and references contained therein. rationale for the formation of 6 would involve initial ionization of 3 to give 7 3 which by virtue of its mode of generation would be in the singlet state. Spin inversion could then occur, converting 7 into the triplet species 8. This triplet might be expected to behave more like a 3 7 singlet 8 triplet H 9 H 6 10 nitrogen cation radical than like a carbonium ion or singlet nitrenium ion. Since nitrogen cation radicals are excellent hydrogen abstractors4 and methanol is a good source of hydrogen atoms, 8 would be expected to react with methanol to yield 9. A second hydrogen abstraction from methanol by 9 would yield 10 which on neutralization with base would give 6.In order to test the validity of this hypothesis we investigated the effect of heavy atom solvents on the product composition observed in the methanolysis of 3. Since heavy atoms are known to enhance the probability of singlet to triplet inversion through coupling of spin and orbital angular momenta,jI6 the amount of 6 should increase in the presence of heavy atoms if our hypothesis is correct. Table I presents the results of our investigation. As shown in Table I the presence of heavy atoms has a very dramatic effect on the product ratios. A comparison of the products from methanol-bromoform and pure methanol showed that the presence of bromoform resulted in a 500-fold change in product ratio! Smaller effects were noted for chlorofor...

Electronic Properties of Nitrenium Ions

“…There has been considerable historical interest in alkyl nitrenium ions, dating back to pioneering experimental studies by Gassman et al 9,19 These workers attempted to generate nitrenium ion 14 from solvolysis of a N-chloramine precursor 13 ( Fig. 6.7).…”

Singlet and triplet states in the reactions of nitrenium ions

“…[11][12][13][14] This follows because nitrenium ions possess two non-bonding electrons and two non-bonding orbitals. This is illustrated in Fig.…”

Reaction of aromatic azides with strong acids: formation of fused nitrogen heterocycles and arylamines

“…To avoid this intermolecular reaction, we propose that it is necessary to use a less acidic medium or to block the para and ortho positions in relation to the azide to allow the intramolecular electrophilic attack of the nitrenium ion upon an ortho-aromatic nucleus; further studies are in course to prove this assertion. Another possible pathway is via the initial singlet nitrenium ion with an extended lifetime which is transformed into a triplet nitrenium ion that may abstract hydrogen atoms 30 from the medium to produce amines or tarry polymers 9c . Thermal decomposition of the same azides gave comparable results in some cases and better or worse than acid decomposition in other cases.…”